CM6903A Datasheet, PDF (10/14 Pages) Champion Microelectronic Corp. – Low Pin Count PFC/PWM CONTROLLER COMBO

CM6903A
Low Pin Count PFC/PWM CONTROLLER COMBO
The I_SENSE Filter is a RC filter. The resistor value of the
I_SENSE Filter is between 100 ohm and 50 ohm. By selecting
R_FILTER equal to 50 ohm will keep the offset of the IEAO less
than 5mV. Usually, we design the pole of I_SENSE Filter at
fpfc/6, one sixth of the PFC switching frequency. Therefore,
the boost inductor can be reduced 6 times without
disturbing the stability. Therefore, the capacitor of the I_SENSE
Filter, C_FILTER, will be around 283nF.
IAC, R_AC, Automatic Slope Compensation, DCM at high line
and light load, and Startup current
There are 4 purposes for IAC pin:
1.) For the leading edge modulation, when the duty
cycle is less than 50%, it requires the similar slope
compensation, as the duty cycle of the trailing
edge modulation is greater than 50%. In the
CM6903A, it is a relatively easy thing to design.
Use an more than 800K ohm resistor, R_AC to
connect IAC pin and the rectified line voltage. It
will do the automatic slope compensation. If the
input boost inductor is too small, the R_AC may
need to be reduced more.
2.) During the startup period, Rac also provides the
initial startup current, 100uA;therefore, the bleed
resistor is not needed.
3.) Since IAC pin with R_AC behaves as a feedforward
signal, it also enhances the signal to noise ratio
and the THD of the input current.
4.) It also will try to keep the maximum input power to
be constant. However, the maximum input power
will still go up when the input line voltage goes up.
Start Up of the system, UVLO, and VREFOK
During the Start-up period, R_AC resistor will provide the start
up current~100uA from the rectified line voltage to IAC pin.
Inside of CM6903A during the start-up period, IAC is
connected to VCC until the VCC reaches UVLO voltage
which is 13V and internal reference voltage is stable, it will
disconnect itself from VCC.
PFC section wakes up after Start up period
After Start up period, PFC section will softly start since
VEAO is zero before the start-up period. Since VEAO is a
slew rate enhanced transconductance amplifier (see figure
3), VEAO has a high impedance output like a current
source and it will slowly charge the compensation net work
which needs to be designed by using the voltage loop gain
equation.
Before PFC boost output reaches its design voltage, it is
around 380V and VFB reaches 2.5V, PWM section is off.
PWM section wakes up after PFC reaches steady state
PWM section is off all the time before PFC VFB reaches
2.45V. Then internal 10mS digital PWM soft start circuit
slowly ramps up the soft-start voltage.
PFC OVP Comparator
PFC OVP Comparator sense VFB pin which is the same the
voltage loop input. The good thing is the compensation
network is connected to VEAO. The PFC OVP function is a
relative fast OVP. It is not like the conventional error amplifier
which is an operational amplifier and it requires a local
feedback and it make the OVP action becomes very slow.
The threshold of the PFC OVP is 2.5V+10% =2.75V with
250mV hysteresis.
Tri-Fault Detect Comparator
To improve power supply reliability, reduce system
component count, and simplify compliance to UL1950 safety
standards, the CM6903A includes Tri-Fault Detect. This
feature monitors VFB (Pin 8) for certain PFC fault conditions.
In case of a feedback path failure, the output of the PFC
could go out of safe operating limits. With such a failure, VFB
will go outside of its normal operating area. Should VFB go
too low, too high, or open, Tri-Fault Detect senses the error
and terminates the PFC output drive.
Tri-Fault detect is an entirely internal circuit. It requires no
external components to serve its protective function.
VCC OVP and generate VCC
For the CM6903A system, if VCC is generated from a source
that is proportional to the PFC output voltage and once that
source reaches 17.9V, PFCOUT, PFC driver will be off.
The VCC OVP resets once the VCC discharges below
16.4V, PFC output driver is enabled. It serves as redundant
PFC OVP function.
Typically, there is a bootstrap winding off the boost inductor.
The VCC OVP comparator senses when this voltage
exceeds 17.9V, and terminates the PFC output drive. Once
the VCC rail has decreased to below 16.4V the PFC output
drive be enabled. Given that 16V on VCC corresponds to
380V on the PFC output, 17.9V on VCC corresponds to an
OVP level of 460V.
It is a necessary to put RC filter between bootstrap winding
and VCC. For VCC=13V, it is sufficient to drive either a
power MOSFET or a IGBT.
2005/01/20
Champion Microelectronic Corporation
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